This invention relates to charged-particle-beam (CPB) exposure devices for transferring a pattern from a self-supporting thin-film mask onto a substrate, such as a wafer.
Mask-based charged-particle-beam (CPB) exposure devices use a charged-particle beam to transfer a desired pattern from a mask to a substrate. The substrate is typically in the form of a charged-particle-sensitive substrate such as a photo-resist-coated wafer or the like.
An example of a conventional CPB exposure device is shown schematically in FIG. 6. The X axis, Y axis, and Z axis are defined as shown in the figure.
In this conventional CPB exposure device, a charged-particle beam in the form of an electron beam EB is emitted from an electron gun 1. The electron beam EB is formed into a constant cross-sectional size and shape by an aperture 3 situated between the electron gun 1 and a mask 2. A position-selection deflector 4 deflects the shaped electron beam EB to irradiate a selected sub-field at a selected position on the mask. The electron beam EB, which has been patterned by passing through the selected sub-field, is then passed through a lens 5 to form a reduced image of the patterned beam at a selected position on a substrate such as a semiconductor wafer 6. The selected position on the wafer 6 is thus exposed, at a particular reduction ratio, with the pattern from the selected sub-field of the mask 2, and the pattern image of the selected sub-field on the mask 2 is transferred to a corresponding position on the wafer 6.
The various other sub-fields on the mask 2 are then sequentially and selectively irradiated by the shaped electron beam EB and exposed on the wafer 6 at respective desired locations. This is accomplished by deflecting the beam with position-selection deflector 4, and by shifting the wafer 6, in the X- and Y-directions, with a substrate stage 7, and by shifting the mask 2 in the X- and Y-directions with a mask stage 8. Patterns formed by irradiation of various individual sub-fields at the mask are positioned on the wafer so as to be aligned and joined or xe2x80x9cstitchedxe2x80x9d together, so that a pattern that is divided into various sub-fields on the mask 2 is formed as a single pattern on the wafer 6. A controller 9 controls the various elements 1, 4, 5, 7, and 8 described above, and a deflector 12, in order to accomplish the pattern transfer.
Note that the substrate stage 7 can generally be moved not only in the X- and Y-directions, but also the Z-direction as well.
Prior to or during the exposure operation described above, device calibrations are performed, such as calibrating the image rotation, image magnification, and the like, in the image-transfer field. This device calibration is typically accomplished by irradiating the electron beam EB onto marks formed at a specific location on the mask 2. An image of the marks on the mask is formed on the wafer 6. The image, formed by the charged-particle beam (electron beam) EB, causes back-scattered electrons and/or other radiation to emanate from corresponding marks on the wafer 6. A detector 10 is employed to detect the electrons and/or other radiation emanating from the corresponding marks on the wafer 6. The controller 9 then calculates control parameters and the like for the lens 5, the deflector 12, and/or similar exposure-device components, based on the detected radiation from the marks on the wafer 6. The control parameters are then saved and/or used to control the wafer stage 7 and/or the mask stage 8.
Also prior to or during the normal exposure operation described above, the mask 2 either is initially installed to the position for exposure, or is swapped and loaded by a mask loader 11. The mask 2 is then initially aligned (also referred to as rough alignment), using alignment marks previously formed on the mask, by a process similar to the process used for device calibration. Namely, the position-selection deflector 4 deflects the electron beam EB so as to irradiate one or more locations, at the plane of the mask 2, where the alignment marks should be positioned. The detector 10 is then used to detect back-scattered electrons and/or other radiation emanating from corresponding marks on the wafer 6 or on the wafer stage 7. The controller 9 then controls the mask stage 8 and/or the wafer stage 7, based on the detected radiation, so as to eliminate any positional shift at the wafer between the image of the alignment marks and the marks on the wafer or wafer stage, thereby rough-aligning the mask.
Note that description of other typical elements generally employed in the illumination system of the electron-beam exposure device to illuminate the aperture 3 and the mask 2 have been omitted above for simplicity. Such elements are known in the art and need not be reviewed here.
In a conventional mask-based CPB exposure device like the example described above, the beam is always shaped to a given constant cross-sectional size and shape by the aperture 3. The constant size and shape of the beam is selected so as to match the size of the sub-fields to be irradiated at the wafer.
Although an exposure device using a mask and having a constant beam-shape, determined in the manner described above, has no particular detriment during actual pattern exposure and during use with some types of masks, the inventor has found that problems can arise during use of a self-supporting thin-film type mask. These problems will be explained below, with respect to the use of a self-supporting thin-film type mask as shown in FIG.7.
FIG. 7 shows an example of a known self-supporting thin-film type mask. FIG. 7(a) is a schematic plan view. FIG. 7(b) is a cross-sectional view taken along the line Axe2x80x94Axe2x80x2 in FIG. 7(a). The mask is constructed from a thin film 21 which scatters electrons at a large scattering angle, and a thick grid-shaped frame 22 which supports the thin film 21 at its bottom surface. Since the frame 22 is thick, when the charged-particle beam irradiates the frame 22, the frame absorbs a significant portion of the incident particles. The process of absorbing incident particles heats the frame. Heating the frame deforms the mask, decreasing the transfer precision of the pattern on the mask.
Pattern-formation fields (transfer fields) 32 are smaller than, and positioned within, respective square fields 31 defined by the frame 22. An irradiation field 33 (shown by the cross-hatching) corresponds to the beam cross-sectional size and shape, and is predetermined to be larger than the pattern-formation fields 32, but smaller than the square fields 31. The electron beam, which has been shaped by the aperture 3 of FIG. 6, sequentially irradiates multiple irradiation fields 33 in the mark of FIG. 7(a) (only one irradiation field 33 is shown). While it is not shown in the figure, a desired pattern to be transferred to the wafer 6 is formed by openings or low-scattering-angle areas in each pattern formation field 32 of the thin film 21.
When this type of mask is used in an exposure device, an aperture is provided (not shown in FIG. 6) that blocks the electrons that have been scattered by the non-open and/or non-low-scattering areas of the thin film 21. (Such an aperture would also be used, though not shown, in the devices shown in FIGS. 1, 3, and 4 to be described below.)
If the self-supporting thin-film type mask shown in FIG. 7 is used in the conventional CPB exposure device shown in FIG. 6, there will generally be no problems during normal exposure. However, the inventor has found that the following problems can arise during the device calibration and mask alignment processes discussed above.
First, the problems arising during device calibration will be explained, with reference to FIG. 8. FIG. 8 is a schematic plan view of another portion of the mask shown in FIG. 7, showing irradiation of the electron beam during device calibration.
For use in device calibration, particularly for calibration of image rotation, image magnification, and the like, multiple calibration marks 34 (formed, similar to a pattern feature, by openings or low-scattering-angle areas in the thin film 21) are arranged at selected positions within the transfer field 32. Four such marks are shown in FIG. 8.
It is often desirable to measure some or all of the marks 34 individually, by irradiating each mark individually with the electron beam. For example, the mark uppermost in the figure, mark 34, is shown irradiated individually by the irradiation field 33xe2x80x2 in FIG. 8. Because the irradiation field 33xe2x80x2 is displaced so as to illuminate only one of the calibration marks 34, a portion of the electron beam irradiates the frame 22 of the mask during calibration. As discussed above, the frame 22 thus absorbs a portion of the electron beam, and generates heat, distorting the mask, thereby decreasing the accuracy and precision of the calibration process.
One possible way to prevent this distortion of the mask is to increase the size of the square field 31 defined by the frame members around the marks 34. (Other fields 31 may be kept the same size.) FIG. 9 is a plan view of structure similar to that of FIG. 8, but with such an enlarged square field 31. Elements in FIG. 9 identical or corresponding to elements in FIGS. 7 and 8 have the same respective reference characters.
The size of the square field 31 of FIG. 9 is increased sufficiently relative to that of FIGS. 7 and 8 such that the irradiation field 33xe2x80x2 does not include the frame 22, as shown in FIG. 9. Heating of the frame is thus avoided, but the surface area of the square field 31 that is within the frame 22 (i.e., the area not directly supported by the frame) is then about four times larger than the corresponding area in FIGS. 7 and 8. As a result of the larger unsupported area, the accuracy of the positioning of the marks 34 themselves decreases, thereby decreasing the accuracy of device calibration performed therewith.
The problems arising during initial mask alignment will now be explained with reference to FIG. 10. FIG. 10 is a schematic plan view of still another portion of the mask of FIG. 7. FIG. 10 shows the position of an irradiation field 33xe2x80x3 on a mask that is in the initial position as loaded by the mask loader 11. FIG. 10(a) shows the position of the irradiation field 33xe2x80x3 when the loading accuracy of the mask loader 11 was high, and FIG. 10(b) shows the position of the irradiation field 33xe2x80x3 when the loading accuracy of the mask loader 11 was low.
A mask-alignment mark 35 (formed as an opening or a low-scattering angle area in the thin film 21) is situated in the transfer field. As mentioned above, when the mask has been loaded, the mark 35 is measured by irradiating the area around and including the location where the mark 35 should be positioned. Back-scattered electrons or other radiation from a corresponding mark on the wafer 6 or on the wafer stage 7 (e.g., a mark made from metallic thin film) are detected by the detector 10, and the relative alignment of the mask and the wafer or the wafer stage is adjusted accordingly.
In the case of the mark 35 on the mask, if the loading precision of the immediately prior loading step is high, the frame 22 of the mask will not be irradiated by the electron beam, as shown in FIG. 10(a). But if the loading precision of the loading step is low, the frame 22 will be irradiated by the electron beam, as shown in FIG. 10(b), and the frame 22 will absorb a portion of the beam, generating heat and deforming the mask, thereby decreasing the accuracy and precision of the initial mask alignment process.
The size of the square field 31 that is surrounded by the frame may be enlarged so as to prevent the irradiation field 33xe2x80x3 irradiated by the electron beam from touching the frame even if the loading precision is low. As discussed above, however, increasing the size of square field 31 decreases the accuracy of the position of the features in the field such as alignment mark 35. Alternatively, loading precision may be increased sufficiently to avoid the degree of inaccuracy shown in FIG. 10(b). Increasing loading precision is difficult, however.
In recognition of the above problems arising during device calibration and initial mask alignment, the present invention provides a mask-based CPB exposure device that can change the size and/or shape of the irradiated field at the mask. This invention thus allows the use of self-supporting thin-film type masks, while avoiding the requirement that the mask field in which the device calibration marks are formed be made an especially large size. This invention similarly allows the use of self-supporting thin-film type masks (scattering masks) while avoiding the requirement either that the mask field in which the mask alignment marks are formed be made an especially large size, or that the loading precision of the mask loader be increased.
Means for changing the size and/or shape of the irradiated field at the mask may take various forms.
In one embodiment, the illumination system includes two apertures arranged in succession along the beam path with a deflector positioned between. The deflector deflects the beam, which has been shaped by the first aperture, so as to be selectively shaped and/or sized by the second aperture.
In another embodiment, the illumination system includes a beam-shaping aperture and a variable-magnification lens for projecting the aperture, at a selected magnification, onto the mask. Varying the magnification of the lens varies the size of the beam cross-section at the mask.
In yet another embodiment, the illumination system includes an aperture array positioned in the beam path and having multiple apertures differing from each other in size and/or shape. A means for causing the beam to pass through a selected one of the apertures is also included, and may take the form of a shift mechanism for mechanically shifting the aperture array, or of a deflector for deflecting the beam to pass through the selected aperture. A pre-shaping aperture may be included in the beam path upstream of the aperture array so as to pre-shape the beam to a size and shape just larger than the largest aperture of the aperture array, allowing close spacing of the apertures in the array.
In each of the above-mentioned embodiments, a mask-based charged-particle-beam exposure device includes an illumination system that provides the capability of varying the beam cross-sectional size and/or shape at the mask. This capability may be used to perform calibration of the device, or initial (xe2x80x9croughxe2x80x9d) alignment of a mask, using a beam having a smaller cross-section than the cross-section of a beam used for normal exposure. This reduces the size of the irradiation field at the mask. The effects of mask heating can thus be avoided, without increasing the unsupported area of the mask around the measurement marks, and without increasing the performance of the mask loader. (Increases in the beam cross-section at the mask could also be caused, if desired.)